1. Field of the Invention
The invention is concerned generally with a deceleration apparatus capable of filtering out neutral particles, and more particularly, a deceleration apparatus capable of producing either a short spot beam or a tall ribbon beam with good beam angle control and smooth profiles.
2. Description of Related Art
Ion implantation is a process used to introduce into a target substrate atoms or molecules, generally referred to as dopants, to make materials with useful properties. Of particular interest, ion implantation is a common process used in making modern integrated circuits. Recently, interest has focused on generating ribbon beams of over 300 mm in size containing milliampere currents of ions at energies as low as 200 eV.
The highest beam currents are obtained by decelerating the ion beam immediately prior to the target; however, this practice has several known disadvantages. One disadvantage is that the deceleration tends to modify the trajectories, magnifying any angular errors and making control of uniformity in a ribbon beam more difficult. Another disadvantage is that a portion of the ions is neutralized by charge-exchange processes with residual gas atoms and molecules and, as a result, is not decelerated. These ions penetrate into the silicon much further than is intended, and this deep penetration of some of the dopant ions interferes with the intended process; furthermore, since neutralization depends on system pressure within the vacuum system, it is difficult to maintain constant conditions from day to day, and the level of contamination is not sufficiently constant to be tolerated.
Many recent innovations to ribbon-beam implanting systems are discussed or disclosed in U.S. Pat. No. 7,902,527, which is incorporated herein by reference. Key content of this patent is summarized below.
Some implanters use a lens to halt the divergence of the ion beam on reaching the requisite major dimensional size, and to collimate it, i.e. render it parallel. A suitable lens may use magnetic or electric fields, may generate a quadrupole field, and must have a beam passage of high aspect ratio to conform generally to the ribbon shape of the ion beam.
In certain circumstances such as when using high-current low energy beams it may not be possible to reliably deliver a ribbon beam that is sufficiently uniform, so the '527 patent discloses a two-mode implantation system. This comprises two multipole lenses after the analyzing magnet. In a first mode, the currents in the coils of one multipole lens can be controlled responsive to a measurement of the ion beam profile to control the current density in this beam profile. The ion beam is allowed to continue as a ribbon-shaped beam whose major dimension exceeds a dimension of the workpiece. The workpiece is then translated through this ion beam along a single path, one or more times, to implant a desired uniform dose of ions into its surface. In a second mode, the currents in the coils of a first multipole lens are excited so as to generate a quadrupole magnetic field which causes the ribbon ion beam to converge in its major dimension, thereby generating at a downstream location a beam spot which is smaller in both dimensions than either dimension of the workpiece—referred to hereinafter as a ‘spot beam’. The workpiece is then translated in a reciprocating path in two dimensions through the ion beam, so as to implant a uniform dose of ions into its surface by implanting a succession of partially overlapping stripes. This is referred to as two-dimensional scanning.
It is generally desirable to minimize the number of passes required in order to achieve a specified dose uniformity. Commonly a standard deviation of 1% or less of the overall dose is an acceptable uniformity. The uniformity achievable depends upon the shape of the ion beam, specifically in its projection in the direction of the striping, or the ‘slow scan direction’. The profile of the beam needs to be a smooth ‘bell curve’. If it contains spikes or valleys in the profile, these will cause an increase in non-uniformity, which can be offset by a large increase in the number of stripes. However, increasing the number of beam passes will decrease the throughput, so is less economically viable.
The second mode is likely to be advantageous when using high-current, low-energy beams (for example greater than 1 mA at energies below 3 keV), under which conditions space-charge and other effects make positive control of the uniformity of the current in a beam more difficult. The first mode requires slower motions and is likely to deliver higher processing throughput at energies where satisfactory control of the ion beam profile can be achieved. The currents in the multipole lens in either mode may be adjusted to fine-tune the beam current density profile of the beam, even though at low energy this control is insufficient to ensure a uniform implant in one pass with a ribbon beam. In the second mode, this may be valuable to generate a bell-curve profile.
The '527 patent further discloses a second lens after the first multipole lens, whose function is to collimate the ion beam. This is particularly important for the first operating mode, i.e. the ribbon-beam case, where systematic variation in the implant angle across the face of the workpiece would otherwise occur. It is also of value to reduce the range of angular variation in the ion beam in the second mode.
The '527 patent further discloses optional means of deceleration or acceleration of the ion beam using a bent ion beam path, to deliver high beam currents at low energies while filtering out contaminants with the wrong energy, for use in ion implantation in either the ribbon-beam or 2D scan beam modes. The beam is bent through an angle that differs by a small amount from standard conditions, then the ion beam is decelerated by means of a set of electrodes that superimpose two opposed successive sideways components of electric field on the deceleration field, so that the ion beam is deflected in an s-shaped bend, the deflections each amounting to an angle of at least 10 degrees, and providing a lateral displacement of several times the width of the ion beam, returning it to its original path. By providing beam stops on either side of the beam, the only ions transmitted are those with the correct charge and energy, so contaminants with the wrong energy or charge can be removed. Such contaminants include neutral atoms formed from beam ions by charge exchange with the residual gas, and since the cross-sections for some charge-exchange reactions peak at beam energies below 1 keV, this becomes very important. This deceleration means has been described as a ‘chicane’ deceleration scheme.